The present invention relates to an optical signal receiver with improved self-adjusting capabilities.
Referring to FIG. 1, a conventional optical signal receiver comprises a photodiode 1, an n-channel junction field-effect transistor (FET) 2, and a transimpedance amplifier 3 including a feedback resistor rfb. The photodiode 1 is illuminated by the received optical signal, and generates a varying signal current at node S1. The FET 2 sinks a constant bias current. The transimpedance amplifier 3 receives the net difference between the signal current and bias current, and generates an output voltage at node S2.
To prevent distortion of the output voltage signal, the bias current must be adjusted so that the net current entering the amplifier 3 varies within the linear operating range of the amplifier 3. Performing this adjustment externally would be inconvenient, and would require readjustment when the operating environment changed, so the receiver preferably has a built-in circuit that adjusts the bias current automatically.
The self-adjusting circuit in a conventional receiver comprises a level-shifting buffer 4, an inverting circuit 5, and an integrating circuit 6 coupled in cascade between the output node S2 and the gate of the FET 2. The level-shifting buffer 4 and inverting circuit 5 are necessary because the amplifier 3 operates with negative gain. The integrating circuit 6 provides the FET 2 with a gate voltage responsive to the average optical signal level. At high average optical signal levels, the gate voltage rises, and the bias current becomes high enough to keep the amplifier 3 from saturating.
To work well, however, the conventional receiver needs to be manufactured to extremely tight tolerances, because the bias current depends on the operating characteristics of the level-shifting buffer 4 and inverting circuit 5, which are sensitive to variations in the fabrication process. Moreover, the operating characteristics of the level-shifting buffer 4 and inverting circuit 5 are sensitive to ambient temperature during operation. In practice, the fabrication process and operating environment cannot always be controlled as closely as necessary, the bias current is sometimes inadequate or over-adequate, and the amplifier 3 sometimes saturates, leading to distortion of the output signal.
Needless to say, using an external adjustment to compensate for variations in the operating characteristics of the level-shifting buffer 4 and inverting circuit 5 would not be a satisfactory solution to this problem.
An object of the present invention is to prevent output distortion in an optical signal receiver.
A further object is to prevent output distortion without requiring external adjustments.
The invented optical signal receiver has a photosensitive element generating a signal current, an amplifier generating an output signal from the signal current, and circuits for automatically adjusting the amplifier by the steps of:
(a) generating two complementary signals from the output signal;
(b) integrating the two complementary signals;
(c) taking the difference between the two integrated signals, thereby obtaining a difference signal; and
(d) adjusting the amplifier according to the difference signal.
Step (d) can be carried out by adjusting a bias current supplied to the amplifier, or by adjusting the gain of the amplifier.
By integrating two complementary signals, the invented optical signal receiver controls the amplifier so as to eliminate output signal distortion in a manner not affected by fabrication-process variations or ambient conditions, without requiring external adjustment.